<p> Tokamaks use magnetic fields to confine plasmas to achieve fusion; they are the leading approach proposed for the widespread production of fusion energy. The sawtooth crash in tokamaks limits the core temperature, adversely impacts confinement, and seeds disruptions. Adequate knowledge of the physics governing the sawtooth crash and a predictive capability of its ramifications has been elusive, including an understanding of incomplete reconnection, i.e., why sawteeth often cease prematurely before processing all available magnetic flux. In this dissertation, we introduce a model for incomplete reconnection in sawtooth crashes resulting from increasing diamagnetic effects in the nonlinear phase of magnetic reconnection. Physically, the reconnection inflow self-consistently convects the high pressure core of a tokamak toward the <i>q</i>=1 rational surface, thereby increasing the pressure gradient at the reconnection site. If the pressure gradient at the rational surface becomes large enough due to the self-consistent evolution, incomplete reconnection will occur due to diamagnetic effects becoming large enough to suppress reconnection. Predictions of this model are borne out in large-scale proof-of-principle two-fluid simulations of reconnection in a 2D slab geometry and are also consistent with data from the Mega Ampere Spherical Tokamak (MAST). Additionally, we present simulations from the 3D extended-MHD code M3D-C¹ used to study the sawtooth crash in a 3D toroidal geometry for resistive-MHD and two-fluid models. This is the first study in a 3D tokamak geometry to show that the inclusion of two-fluid physics in the model equations is essential for recovering timescales more closely in line with experimental results compared to resistive-MHD and contrast the dynamics in the two models. We use a novel approach to sample the data in the plane of reconnection perpendicular to the <i>(m,n)</i>=(1,1) mode to carefully assess the reconnection physics. Using local measures of reconnection, we find that it is much faster in the two-fluid simulations, consistent with expectations based on global measures. By sampling data in the reconnection plane, we present the first observation of the quadrupole out-of-plane magnetic field appearing during sawtooth reconnection with the Hall term. We also explore how reconnection as viewed in the reconnection plane varies toroidally, which affects the symmetry of the reconnection geometry and the local diamagnetic effects. We expect our results to be useful for transport modeling in tokamaks, predicting energetic alpha-particle confinement, and assessing how sawteeth trigger disruptions. Since the model only depends on local diamagnetic and reconnection physics, it is machine independent, and should apply both to existing tokamaks and future ones such as ITER.</p>

<p> Magnetic fields at all scales are prevalent in our universe. However, current cosmological models predict that initially the universe was bereft of large-scale fields. Standard magnetohydrodynamics (MHD) does not permit magnetogenesis; in the MHD Faraday&rsquo;s law, the change in magnetic field <i> B</i> depends on <i>B</i> itself. Thus if <i> B</i> is initially zero, it will remain zero for all time. A more accurate physical model is needed to explain the origins of the galactic-scale magnetic fields observed today. In this thesis, I explore two velocity-driven mechanisms for magnetogenesis in 2-fluid plasma. The first is a novel kinematic &lsquo;battery&rsquo; arising from convection of vorticity. A coupling between thermal and plasma oscillations, this non-relativistic mechanism can operate in flows that are incompressible, quasi-neutral and barotropic. The second mechanism results from inclusion of thermal effects in relativistic shear flow instabilities. In such flows, parallel perturbations are ubiquitously unstable at small scales, with growth rates of order with the plasma frequency over a defined range of parameter-space. Of these two processes, instabilities seem far more likely to account for galactic magnetic fields. Stable kinematic effects will, at best, be comparable to an ideal Biermann battery, which is suspected to be orders of magnitude too weak to produce the observed galactic fields. On the other hand, instabilities grow until saturation is reached, a topic that has yet to be explored in detail on cosmological scales. In addition to investigating these magnetogenesis sources, I derive a general dispersion relation for three dimensional, warm, two species plasma with discontinuous shear flow. The mathematics of relativistic plasma, sheared-flow instability and the Biermann battery are also discussed.</p>

<p>This thesis demonstrates a measurement of a plasma fluctuation velocity-space cross-correlation matrix using laser induced fluorescence. The plasma fluctuation eigenmode structure on the ion velocity distribution function can be empirically determined through singular value decomposition from this measurement. This decomposition also gives the relative strengths of the modes as a function of frequency. Symmetry properties of the matrix quantify systematic error. The relation between the eigenmodes and plasma kinetic fluctuation modes is explored. A generalized wave admittance is calculated for these eigenmodes. Since the measurement is a localized technique, it may be applied to plasmas in which a single point measurement is possible, multipoint measurements may be difficult, and a velocity sensitive measurement technique is available.

<p> The absence of a protecting magnetic field, such as the dipole magnetic field around Earth, makes the interaction of solar wind with unmagnetized objects particularly interesting. Long-term evolution of the object&rsquo;s surface and atmosphere is closely tied to its interaction with the outer space environment. The ionospheric plasma layer around unmagnetized objects acts as an electrically conducting transition layer between lower atmospheric layers and outer space. This study considers two distinct types of unmagnetized objects: Titan and comet 67P/Churyumov-Gerasimenko (67P/CG). For many years, Titan has been a key target of the National Aeronautics and Space Administration (NASA) Cassini mission investigations; and the European Space Agency (ESA) Rosetta spacecraft explored comet 67P/CG for more than two years. </p><p> Ionospheric composition and primary ion production rate profiles for Titan are modeled for various solar activity conditions. Photoionization is the main source of ion production on the dayside; on the nightside, electron-impact ionization is the main ionization source. This dissertation uses model results and in-situ measurements by the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir Probe (LP) onboard the Cassini spacecraft to show that while the solar activity cycle impacts the primary ion species significantly, there is little effect on heavy ion species. Solar cycle modulates the Titan&rsquo;s ionospheric chemistry. The solar cycle effects of on each ion species are quantified n this work. In some cases the solar zenith angle significantly overshadows the solar cycle effects. How each individual ion reacts to changes in solar activity and solar zenith angle is discussed in details. A method to disentangle these effects in ion densities is introduced. </p><p> At comet 67P/CG, the fast-moving solar wind impacts the neutral coma. Two populations of electrons are recognizable in the cometary plasma. These are the hot suprathermal electrons, created by photoionization or electron-impact ionization, and the cold/thermal electrons. Even though photoionization is the dominant source of ion production, electron-impact ionization can be as high as the photoionization for certain solar events. At 3 AU, electron energy spectra from in-situ measurements of the Ion and Electron Sensor (IES) instrument exhibit enhancement of electron fluxes at particular energies. Model-data comparisons show that the flux of electrons is higher than the typical solar wind and pure photoionization fluxes. The probable cause of this enhancement is the ambipolar electric field and/or plasma compression. </p><p> This research also discusses formation of a new boundary layer around the comet near perihelion, similar to the diamagnetic cavity at comet 1P/Halley. At each crossing event to the diamagnetic cavity region, flux of suprathermal electrons with energies between 40 to 250 eV drops. The lower flux of solar wind suprathermal electrons in that energy range can cause this flux drop. </p><p>

<p>The magnetosphere-ionosphere (M-I) transition region is the several thousand--kilometer stretch between the cold, dense and variably resistive region of ionized atmospheric gases beginning tens of kilometers above the terrestrial surface, and the hot, tenuous, and conductive plasmas that interface with the solar wind at higher altitudes. The M-I transition region is therefore the site through which magnetospheric conditions, which are strongly susceptible to solar wind dynamics, are communicated to ionospheric plasmas, and vice versa. We systematically study the influence of geomagnetic storms on energy input, electron precipitation, and ion outflow in the M-I transition region, emphasizing the role of inertial Alfven waves both as a preferred mechanism for dynamic (instead of static) energy transfer and particle acceleration, and as a low-altitude manifestation of high-altitude interaction between the solar wind and the magnetosphere, as observed by the FAST satellite. Via superposed epoch analysis and high-latitude distributions derived as a function of storm phase, we show that storm main and recovery phase correspond to strong modulations of measures of Alfvenic activity in the vicinity of the cusp as well as premidnight. We demonstrate that storm main and recovery phases occur during ~30% of the four-year period studied, but together account for more than 65% of global Alfvenic energy deposition and electron precipitation, and more than 70% of the coincident ion outflow. We compare observed interplanetary magnetic field (IMF) control of inertial Alfven wave activity with Lyon-Fedder-Mobarry global MHD simulations predicting that southward IMF conditions lead to generation of Alfvenic power in the magnetotail, and that duskward IMF conditions lead to enhanced prenoon Alfvenic power in the Northern Hemisphere. Observed and predicted prenoon Alfvenic power enhancements contrast with direct-entry precipitation, which is instead enhanced postnoon. This situation reverses under dawnward IMF. Despite clear observational and simulated signatures of dayside Alfvenic power, the generation mechanism remains unclear. Last, we present premidnight FAST observations of accelerated precipitation that is best described by a kappa distribution, signaling a nonthermal source population. We examine the implications for the commonly used Knight Relation.

No description available.

<p>The state of matter at extremely high pressures and densities is of fundamental interest to many branches of research, including planetary science, material science, condensed matter physics, and plasma physics. Matter with pressures, or energy densities, above 1 megabar (100 gigapascal) are defined as High Energy Density (HED) plasmas. They are directly relevant to the interiors of planets such as Earth and Jupiter and to the dense fuels in Inertial Confinement Fusion (ICF) experiments. To create HEDP conditions in laboratories, a sample may be compressed by a smoothly varying pressure ramp with minimal temperature increase, following the isentropic thermodynamic process. Isentropic compression of aluminum targets has been done using magnetic pressure produced by megaampere, pulsed power currents having ~ 100 ns rise times. In this research project, magnetically driven, cylindrical isentropic compression has been numerically studied. In cylindrical geometry, material compression and pressure become higher than in planar geometry due to geometrical effects. Based on a semi-analytical model for the Magnetized Liner Inertial Fusion (MagLIF) concept, a code called ?SA? was written to design cylindrical compression experiments on the 1.0 MA Zebra pulsed power generator at the Nevada Terawatt Facility (NTF). To test the physics models in the code, temporal progresses of rod compression and pressure were calculated with SA and compared with 1-D magnetohydrodynamic (MHD) codes. The MHD codes incorporated SESAME tables, for equation of state and resistivity, or the classical Spitzer model. A series of simulations were also run to find optimum rod diameters for 1.0 MA and 1.8 MA Zebra current pulses. For a 1.0 MA current peak and 95 ns rise time, a maximum compression of ~ 2.35 (~ 6.3 g/cm</p><p>3) and a pressure of ~ 900 GPa within a 100 ?m radius were found for an initial diameter of 1.05 mm. For 1.8 MA peak simulations with the same rise time, the initial diameter of 1.3 mm was optimal with ~ 3.32 (~ 9.0 g/cm</p><p>3) compression.

The results of nuclear magnetic resonance (NMR) on thin $\sp3\rm{He}$-$\sp4$He mixture films at temperatures 24mK $\leq T \leq$ 650mK which are adsorbed to Nucleopore are reported. The nuclear magnetic susceptibility, the relaxation times T$\sb1$ and T$\sb2$, and the spin diffusion coefficient, D, were measured used pulsed NMR techniques in a 2 Tesla field. The $\sp4$He coverages investigated ranged from 0.137 $\leq n\sb4\leq$ 0.534 atoms A$\sp2$ with a fixed submonolayer $\sp3$He coverage of 0.00746 $\leq n\sb3 \leq$ 0.00749 atoms A$\sp2.$ At $n\sb4$ = 0.391 atoms/A$\sp2$ measurements were taken with the $\sp3$He coverage ranging 0.00749 $\leq n\sb3 \leq$ 0.0179 atoms/A$\sp2$. We present the $\sp3$He magnetization as a function of the $\sp4$He coverage. The magnetization is degenerate for temperatures below the Fermi temperature, T$\sb{F}$, and from the degenerate magnetization the hydrodynamic mass over a range of $\sp4$He coverages is obtained. Variational and density functional descriptions of the film are considered. The diffusion data are seen to rise rapidly, from 10$\sp{-8}$ to 10$\sp{-3}\ \rm{cm}\sp2$/sec, as the $\sp4$He coverage is increased from 0.19 to 0.39 atoms/A$\sp2$, a range of just 2.5 layers. For $T < T\sb{F}$ the temperature dependence of a degenerate Fermi gas is not seen; $D \not= T\sp{-2}$. For all coverages T$\sb1$ is two orders of magnitude larger than T$\sb2$. Two regimes are seen. For coverages $n\sb4 <$ 0.23 the temperature dependence of T$\sb1$ and T$\sb2$ are consistent with $\omega\tau\sb{c} \gg$ 1. A signature of the completion of the second layer of the $\sp4$He is seen in T$\sb{1}$. For coverages $n\sb4 >$ 0.23, $\omega\tau\sb{c} \ll$ 1 and the temperature dependence correlates with the superfluid areal density. Activated behavior is seen which probes higher bound states of the $\sp3$He in the $\sp4$He film.

The transport of heat that results from turbulence is a major factor limiting the temperature gradient, and thus the performance, of fusion devices. We use nonlinear simulations to show that a toroidal equilibrium scale sheared flow can completely suppress the turbulence across a wide range of flow gradient and temperature gradient values. We demonstrate the existence of a bifurcation across this range whereby the plasma may transition from a low flow gradient and temperature gradient state to a higher flow gradient and temperature gradient state. We show further that the maximum temperature gradient that can be reached by such a transition is limited by the existence, at high flow gradient, of subcritical turbulence driven by the parallel velocity gradient (PVG). We use linear simulations and analytic calculations to examine the properties of the transiently growing modes which give rise to this subcritical turbulence, and conclude that there may be a critical value of the ratio of the PVG to the suppressing perpendicular gradient of the velocity (in a tokamak this ratio is equal to q/ε where q is the magnetic safety factor and ε the inverse aspect ra- tio) below which the PVG is unable to drive subcritical turbulence. In light of this, we use nonlinear simulations to calculate, as a function of three parameters (the perpendicular flow shear, q/ε and the temperature gradient), the surface within that parameter space which divides the regions where turbulence can and cannot be sustained: the zero- turbulence manifold. We are unable to conclude that there is in fact a critical value of q/ε below which PVG-driven turbulence is eliminated. Nevertheless, we demonstrate that at low values of q/ε, the maximum critical temperature gradient that can be reached without generating turbulence (and thus, we infer, the maximum temperature gradient that could be reached in the transport bifurcation) is dramatically increased. Thus, we anticipate that a fusion device for which, across a significant portion of the minor radius, the magnetic shear is low, the ratio q/ε is low and the toroidal flow shear is strong, will achieve high levels of energy confinement and thus high performance.

This thesis presents a complete theoretical framework for turbulence and transport in tokamak plasmas. The fundamental scale separations present in plasma turbulence are codified as an asymptotic expansion in the ratio of the gyroradius to the equilibrium scale length. Proceeding order-by- order in this expansion, a framework for plasma turbulence is developed. It comprises an instantaneous equilibrium, the fluctuations driven by gra- dients in the equilibrium quantities, and the transport-timescale evolu- tion of mean profiles of these quantities driven by the fluctuations. The equilibrium distribution functions are local Maxwellians with each flux surface rotating toroidally as a rigid body. Large-scale deviations of the distribution function from a Maxwellian are given by neoclassical theory. The fluctuations are determined by the high-flow gyrokinetic equation, from which we derive the governing principle for gyrokinetic turbulence in tokamaks: the conservation and local cascade of free energy. Transport equations for the evolution of the mean density, temperature and flow ve- locity profiles are derived. These transport equations show how the neo- classical corrections and the fluctuations act back upon the mean profiles through fluxes and heating. This framework is further developed by exploiting the scale separation between ions and the electrons. The gyrokinetic equation is expanded in powers of the electron to ion mass ratio, which provides a rigorous method for deriving the electron response to ion-scale turbulence. We prove that such turbulence cannot change the magnetic topology, and ar- gue that, therefore, the magnetic field lies on fluctuating flux surfaces. These flux surfaces are used to construct magnetic coordinates, and in these coordinates a closed system of equations for the electron response is derived. All fast electron timescales have been eliminated from these equations. Simplified transport equations for electrons in this limit are also derived.